Voltage regulators may be found in all kind of electronic devices such as e.g. power supply circuits for stabilizing the DC voltages used by the processor, the display, or other components of an electronic device. Two prominent classes of voltage regulators are linear regulators and switching regulators. Linear regulators are based on transistors that operate in their linear region, whereas switching regulators are based on transistors forced to act as an on/off switches. For both classes however, over voltage protection OVP mechanisms may be required to prevent overvoltages at the outputs of the voltage regulators. Overvoltages may be the result of transients occurring when a load is connected or disconnected from the output of the voltage regulator. Overvoltages may cause damage to the voltage regulator and the connected circuitry or unacceptable acoustic noise e.g. in audio applications.
Typically, a static OVP device is coupled to the output of a voltage regulator. FIG. 1 illustrates a regulation system receiving an input voltage VIN and being configured to regulate the output voltage VOUT towards the voltage regulation level. In FIG. 1, a Zener diode is connected between the output of a regulation system and ground. In the exemplary scenario, the Zener diode implements a static OVP device which prevents the output voltage VOUT of the regulation system from exceeding a predetermined threshold value. Traditionally, the static OVP device is always enabled and might influence the transient response of the system leading to decreased efficiency. The threshold value of the static OVP device is set to a fixed value which has to be much higher than the voltage regulation level of the regulation system. If the threshold value of the static OVP is set too low, the regulation operation of the regulation system may be affected. This will not only affect the symmetry of the transient response, but will also lead to a decreased efficiency of the overall system.
In case the load is disconnected from the output of the regulation system, a voltage overshoot may occur which is clamped by the static OVP device. The OVP device will then sink excess current to ground to prevent the regulation system from being damaged. For example, the static OVP device may be implemented as a regulated NMOS device able to sink the excess current from the output of the regulation system. Alternatively, the static OVP device may be implemented as a corresponding PMOS device.
Usually, an output capacitor is connected in parallel to the static OVP device between the output of the regulation system and ground. In audio systems, it is of particular interest to minimize the voltage change across the output capacitor to limit the acoustic noise generation. The higher the voltage change across the output capacitor, the higher the mechanical noise generated by the capacitor. One possible solution to this problem is to increase the capacitance of the output capacitor in order to reduce the voltage change caused by load transients. This solution, however, entails the disadvantage that—due to the increased area of the output capacitor—the audio noise is increased again, thereby offsetting the benefits of the reduced voltage change.